U.S. patent application number 13/523235 was filed with the patent office on 2013-12-19 for method of controlling a mobile heating device.
This patent application is currently assigned to Webasto AG. The applicant listed for this patent is Christian Beacker, Michael SCHWANECKE, Steffen Weber, Martin Zoske. Invention is credited to Christian Beacker, Michael SCHWANECKE, Steffen Weber, Martin Zoske.
Application Number | 20130337388 13/523235 |
Document ID | / |
Family ID | 49756214 |
Filed Date | 2013-12-19 |
United States Patent
Application |
20130337388 |
Kind Code |
A1 |
SCHWANECKE; Michael ; et
al. |
December 19, 2013 |
Method of controlling a mobile heating device
Abstract
A method of controlling a mobile heating device which is adapted
for generating heat by combustion of fuel with combustion air is
provided, the method comprising the steps: operating the mobile
heating device with a predetermined fuel supply rate and a
predetermined combustion air supply rate, monitoring the signal of
at least one temperature sensor over time, temporarily changing at
least one of the fuel supply rate and the combustion air supply
rate, analyzing the response in the signal of the at least one
temperature sensor, and based on the response, operating the mobile
heating device with at least one of an adapted fuel supply rate and
an adapted combustion air supply rate.
Inventors: |
SCHWANECKE; Michael;
(Schoengeising, DE) ; Zoske; Martin; (Weilheim,
DE) ; Weber; Steffen; (Anklam, DE) ; Beacker;
Christian; (Fuerstenfeldbruck, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SCHWANECKE; Michael
Zoske; Martin
Weber; Steffen
Beacker; Christian |
Schoengeising
Weilheim
Anklam
Fuerstenfeldbruck |
|
DE
DE
DE
DE |
|
|
Assignee: |
Webasto AG
Stockdorf/Gauting
DE
|
Family ID: |
49756214 |
Appl. No.: |
13/523235 |
Filed: |
June 14, 2012 |
Current U.S.
Class: |
431/12 ;
431/75 |
Current CPC
Class: |
F23N 2225/10 20200101;
F23N 5/003 20130101; F23N 1/02 20130101 |
Class at
Publication: |
431/12 ;
431/75 |
International
Class: |
F23N 1/02 20060101
F23N001/02 |
Claims
1. A method of controlling a mobile heating device which is adapted
for generating heat by combustion of fuel with combustion air, the
method, comprising the steps of: operating the mobile heating
device with a predetermined fuel supply rate and a predetermined
combustion air supply rate; monitoring a signal from at least one
temperature sensor over time; temporarily changing at least one of
the fuel supply rate and the combustion air supply rate; analyzing
a response in the signal from the at least one temperature sensor;
and based on the response, operating the mobile heating device with
at least one of an adapted fuel supply rate and an adapted
combustion air supply rate.
2. The method according to claim 1, wherein the step of operating
the mobile heating device with the at least one of an adapted fuel
supply rate and an adapted combustion air supply rate comprises the
step of operating the mobile heating device with an adapted
air-fuel ratio.
3. The method according to claim 2, wherein the step of analyzing
the response comprises attributing the response to one of an
initial .lamda. value which is equal to or higher than 1 and an
initial .lamda. value lower than 1.
4. The method according to claim 3, wherein the step of operating
the mobile heating device with an adapted air-fuel rate comprises
the step of operating the mobile heating device with an increased
air-fuel ratio if the initial .lamda. value is found to be lower
than 1.
5. The method according to claim 1, wherein the step of temporarily
changing the at least one of the fuel supply rate and the
combustion air supply rate further comprises the step of abruptly
changing the at least one of the fuel supply rate and the
combustion air supply rate for a predetermined time.
6. The method according to claim 1, wherein the step of temporarily
changing the at least one of the fuel supply rate and the
combustion air supply rate further comprises the step of changing
one of the fuel supply rate and the combustion air supply rate in
the manner of a square wave signal.
7. The method according to claim 1, wherein the step of temporarily
changing the at least one of the fuel supply rate and the
combustion air supply rate further comprises the step of abruptly
changing the air-fuel ratio.
8. The method according to claim 1, wherein the step of monitoring
at least one temperature sensor comprises the step of monitoring an
exhaust gas temperature sensor for the step of measuring the
temperature of exhaust gas discharged from a combustion chamber of
the mobile heating device.
9. The method according to claim 1, wherein the step of temporarily
changing at least one of the fuel supply rate and the combustion
air supply rate comprises the step of temporarily reducing one of
the fuel supply rate and the combustion air supply rate by a factor
of 0.67 or lower.
10. The method according to claim 1, wherein the step of
temporarily changing at least one of the fuel supply rate and the
combustion air supply rate comprises the step of temporarily
enhancing one of the fuel supply rate and the combustion air supply
rate by a factor of 1.3 or higher.
11. A mobile heating device comprising: a combustion chamber for
combusting fuel with combustion air; a fuel supply for supplying
fuel to the combustion chamber with a fuel supply rate; a
combustion air supply for supplying combustion air to the
combustion chamber with a combustion air supply rate; at least one
temperature sensor for detecting a temperature of the mobile
heating device, and a control unit for controlling operation of the
mobile heating device, the control unit being adapted such that it
operates the mobile heating device with a predetermined fuel supply
rate and a predetermined combustion air supply rate, monitors the
signal of the at least one temperature sensor over time,
temporarily changes at least one of the fuel supply rate and the
combustion air supply rate, analyzes the response in the signal of
the at least one temperature sensor; and based on the response,
operates the mobile heating device with at least one of an adapted
fuel supply rate and an adapted combustion air supply rate.
12. The mobile heating device according to claim 11, wherein the
mobile heating device operates with at least one of an adapted fuel
supply rate and an adapted combustion air supply rate comprises the
mobile heating device operating with an adapted air-fuel ratio.
13. The mobile heating device according to claim 12, wherein the
device analyzes the response by attributing the response to one of
an initial 2 value that is equal to or higher than 1 and an initial
2 value lower than 1.
14. The mobile heating device according to claim 13, wherein the
mobile heating device operating with an adapted air-fuel ratio
comprises operating the mobile heating device with an increased
air-fuel ratio if the initial .lamda. value is found to be lower
than 1.
15. The mobile heating device according to claim 11, wherein the at
least one temperature sensor comprises an exhaust gas temperature
sensor for measuring the temperature of exhaust gas discharged from
the combustion chamber.
Description
[0001] The present invention relates to a method of controlling a
mobile heating device and to a mobile heating device.
[0002] Heating devices for mobile use or mobile applications (in
the following: mobile heating devices) are particularly used in
vehicles as parking heaters or additional heaters.
[0003] In such heating devices, typically fuel and combustion air
is combusted in order to generate heat. To achieve this, mobile
heating devices typically comprise a combustion chamber in which
combustion takes place. With the requirement to develop vehicles
showing reduced emissions, a demand for developing mobile heating
devices releasing fewer emissions comes along. In general, the
amount of emissions correlates with the ratio of combustion air to
fuel. The air-fuel-ratio AFR is the ratio between the mass of air
and the mass of fuel in the mixture which is combusted. Commonly a
combustion process is categorized in relation to a stoichiometric
combustion in which exactly the mass of air is present which is
need for entire combustion of the fuel. To this end, the ratio
.lamda. (called .lamda. value in the following) of the actual
air-fuel-ratio to the stoichiometric air-fuel-ratio
(.lamda.=AFR.sub.actual/AFR.sub.stoichiometric) is used to
characterize the actual combustion taking place. Thus, .lamda.>1
defines a combustion with excess of combustion air, .lamda.<1
defines a combustion with deficient combustion air, and .lamda.=1
defines stoichiometric combustion. In particular, a combustion
process with .lamda.<1 comes along with increased emissions,
since the fuel cannot be completely combusted. Further, the
temperature which can be attained in the combustions chamber has a
maximum at .lamda.=1 and becomes lower to higher or lower values of
.lamda.. Thus, it is an aim to operate mobile heating devices in a
range of .lamda.>1 in order to achieve an efficient combustion
with low emissions. Typically mobile heating devices are designed
to operate in the range 1.2.ltoreq..lamda..ltoreq.1.8. However, in
applications of mobile heating devices, problems in the supply of
combustion air can occur which result in reduced mass flow of
combustion air and thus undesired air-fuel-ratios. Mobile heating
devices typically do not comprise a lambda sensor (air-fuel ratio
gauge) and typically the mass flow of fuel and the mass flow of
combustion air are not directly measured either. As a consequence,
undesired air-fuel ratios cannot easily be detected.
[0004] It is an object of the present invention to provide an
improved method of controlling a mobile heating device and an
improved mobile heating device.
[0005] The object is attained by a method of controlling a mobile
heating device which is adapted for generating heat by combustion
of fuel with combustion air, the method comprising the steps:
operating the mobile heating device with a predetermined fuel
supply rate and a predetermined combustion air supply rate;
monitoring the signal of at least one temperature sensor over time;
temporarily changing at least one of the fuel supply rate and the
combustion air supply rate; analyzing the response in the signal of
the at least one temperature sensor; and based on the response,
operating the mobile heating device with at least one of an adapted
fuel supply rate and an adapted combustion air supply rate.
[0006] By temporarily changing at least one of the fuel supply rate
and the combustion air supply rate and analyzing the response in
the signal of the temperature sensor, information about the actual
air-fuel ratio can easily be deduced without requiring a costly
lambda sensor and without measuring the mass flow of fuel and the
mass flow of combustion air. Thus, if it is found based on the
analyzed response that the mobile heating device is operated with
an undesired .lamda. value, the mobile heating device can be
operated with at least one of an adapted fuel supply rate and an
adapted combustion air supply rate such that combustion is shifted
to the desired air-fuel ratio. If analysis of the response results
in that the mobile heating device is already operated with the
desired air-fuel ratio, of course neither the fuel supply rate nor
the combustion air supply rate is changed. As a consequence, the
method of controlling a mobile heating device enables operating a
mobile heating device more reliable and with reduced emissions.
Further, plugging of a combustion air supply line can quickly and
easily be detected in which case the mobile heating device can be
shut down, for example. Preferably, temporarily changing at least
one of the fuel supply rate and the combustion air supply rate
comprises temporarily changing the air-fuel ratio. Preferably, the
fuel supply rate is temporarily changed while the combustion air
supply rate is maintained unchanged. In particular, the method can
be realized such that, based on the response, the mobile heating
device is operated with at least one of an adapted fuel supply rate
and an adapted combustion air supply rate only if analyzing the
response results in that an undesired air-fuel ratio is
present.
[0007] According to a further development, operating the mobile
heating device with at least one of an adapted fuel supply rate and
an adapted combustion air supply rate comprises operating the
mobile heating device with an adapted air-fuel ratio. In this case,
the mobile heating device can conveniently be operated with a
desired X, value.
[0008] According to a further development, analyzing the response
comprises attributing the response to one of an initial .lamda.
value which is equal to or higher than 1 and an initial .lamda.
value lower than 1. In this case, based on the result it can easily
be decided in which way a ratio of the combustion air supply rate
to the fuel supply rate should be changed in order to operate the
mobile heating device with low emissions.
[0009] According to a further development, operating the mobile
heating device with an adapted air-fuel rate comprises: if the
initial .lamda. value is found to be lower than 1, operating the
mobile heating device with an increased air-fuel ratio. In this
case, the mobile heating device can conveniently be operated with
reduced emissions. As an alternative, if the initial A, value is
found to be lower than 1 it is also possible to shut down the
mobile heating device in order to prevent operation with increased
emissions.
[0010] According to a further development, temporarily changing the
at least one of the fuel supply rate and the combustion air supply
rate comprises abruptly changing the at least one of the fuel
supply rate and the combustion air supply rate for a predetermined
time. In this context, temporarily changing means that the
respective supply rate is changed only for a predetermined period
and thereafter is returned to the previous value. Abruptly changing
the fuel supply rate or the combustion air supply rate results in a
clear response in the signal of the temperature sensor which can
easily be detected and analyzed.
[0011] According to a further development, temporarily changing the
at least one of the fuel supply rate and the combustion air supply
rate comprises changing one of the fuel supply rate and the
combustion air supply rate in the manner of a square wave signal.
In this case, a particularly clear response is attained. A
particularly clear response can be attained if the fuel supply rate
is changed in this manner.
[0012] According to a further development, temporarily changing the
at least one of the fuel supply rate and the combustion air supply
rate comprises abruptly changing the air-fuel ratio. An abruptly
changed air-fuel ratio results in a particularly easily detectable
response.
[0013] According to a further development, the at least one
temperature sensor comprises an exhaust gas temperature sensor for
measuring the temperature of exhaust gas discharged from a
combustion chamber of the mobile heating device. Since the signal
of an exhaust gas sensor correlates with and follows the
temperature in the combustion chamber, analyzing the response in
the signal of an exhaust gas sensor allows particularly reliable
analysis. For example, the exhaust gas sensor may be arranged
directly downstream of the combustion chamber, e.g. at an entry of
a heat exchanger. However, it is also possible that the exhaust gas
sensor is arranged further downstream, e.g. even downstream of the
heat exchanger, for example in an exhaust gas outlet of the mobile
heating device.
[0014] According to a further development, temporarily changing at
least one of the fuel supply rate and the combustion air supply
rate comprises temporarily reducing one of the fuel supply rate and
the combustion air supply rate by a factor of 0.67 or lower.
Reducing by a factor of 0.67 means that a supply rate is applied
which corresponds to the initial supply rate multiplied by a factor
of at most 0.67. In this case, a pronounced response which can
clearly be attributed to an initial .lamda. value above or below 1
is achieved.
[0015] According to a further development, temporarily changing at
least one of the fuel supply rate and the combustion air supply
rate comprises temporarily enhancing one of the fuel supply rate
and the combustion air supply rate by a factor of 1.3 or higher.
Again, preferably the fuel supply rate is temporarily changed in
order to achieve a particularly pronounced response. Temporarily
enhancing the respective supply rate by a factor of 1.3 or higher
means that a supply rate is applied which corresponds to the
initial supply rate multiplied by a factor of at least 1.3.
[0016] The object is also attained by a mobile heating device
comprising: a combustion chamber for combusting fuel with
combustion air; a fuel supply for supplying fuel to the combustion
chamber with a fuel supply rate; a combustion air supply for
supplying combustion air to the combustion chamber with a
combustion air supply rate; at least one temperature sensor for
detecting a temperature of the mobile heating device; and a control
unit for controlling operation of the mobile heating device,
wherein the control unit is adapted such that it: operates the
mobile heating device with a predetermined fuel supply rate and a
predetermined combustion air supply rate; monitors the signal of
the at least one temperature sensor over time; temporarily changes
at least one of the fuel supply rate and the combustion air supply
rate; analyzes the response in the signal of the at least one
temperature sensor; and based on the response, operates the mobile
heating device with at least one of an adapted fuel supply rate and
an adapted combustion air supply rate. The thus-adapted mobile
heating device achieves the advantages which have been described
above with regard to the method of controlling a mobile heating
device. In particular, by temporarily changing at least one of the
fuel supply rate and the combustion air supply rate and analyzing
the response in the signal of the temperature sensor, information
about the actual air-fuel ratio can easily be deduced without
requiring a costly lambda sensor and without measuring the mass
flow of fuel and the mass flow of combustion air. Thus, based on
the analyzed response, the mobile heating device can be operated
with at least one of an adapted fuel supply rate and an adapted
combustion air supply rate such that combustion is shifted to the
desired air-fuel ratio. As a consequence, the mobile heating device
can be operated more reliable and with reduced emissions. Further,
plugging of a combustion air supply line can quickly and easily be
detected in which case the mobile heating device can be shut down.
Preferably, temporarily changing at least one of the fuel supply
rate and the combustion air supply rate comprises temporarily
changing the air-fuel ratio. Preferably, the fuel supply rate is
temporarily changed while the combustion air supply rate is
maintained unchanged.
[0017] In the present context, the term mobile heating device is
used to characterize a heating device which is adapted such that it
is suited for mobile applications. This means that the heating
device is transportable (e.g. incorporated into a vehicle or placed
therein for transport purposes) and is not solely adapted for a
permanent stationary use as is the case for a domestic heating
device, for example. The mobile heating device can in particular be
adapted for heating a vehicle or a temporarily stationary
application such as big tents or mobile cabins (welfare cabins).
Thus, the term mobile heating device is also used for a heating
device which is adapted for fixed installation in a mobile device
(such as a vehicle or a mobile cabin). In particular, the mobile
heating device can be adapted as a parking heater or additional
heater for a motor vehicle.
[0018] According to a further development, operating the mobile
heating device with at least one of an adapted fuel supply rate and
an adapted combustion air supply rate comprises operating the
mobile heating device with an adapted air-fuel ratio. In this case,
the mobile heating device is adapted to operate with a desired
.lamda. value.
[0019] According to a further development, analyzing the response
comprises attributing the response to one of an initial .lamda.
value which is equal to or higher than 1 and an initial .lamda.
value lower than 1. In this case, the control unit is adapted to
distinguish between operation resulting in undesirably high
emissions and the intended low emission operation and can initiate
appropriate countermeasures in the case of operation with high
emissions.
[0020] According to a further development, operating the mobile
heating device with an adapted air-fuel ratio comprises: if the
initial .lamda. value is found to be lower than 1, operating the
mobile heating device with an increased air-fuel ratio. In this
case, if combustion with high emissions is found, the operation of
the mobile heating device is adjusted to lower emission
operation.
[0021] Instead of operating the heating device with an increased
air-fuel ratio it is also possible to adapt the control unit such
that it shuts down the heating device in the case of an initial
.lamda. value which is below 1.
[0022] According to a further development, the at least one
temperature sensor comprises an exhaust gas temperature sensor for
measuring the temperature of exhaust gas discharged from the
combustion chamber. The exhaust gas temperature sensor allows very
fast and efficient analysis since the temperature of the exhaust
gas correlates well with the temperature in the combustion
chamber.
[0023] Further features and advantages will become apparent from
the following description of an embodiment which will be described
with reference to the enclosed drawings.
[0024] FIG. 1 is a schematic illustration of a mobile heating
device.
[0025] FIG. 2 is a schematic illustration of a typical response
according to the embodiment in the case of an initial .lamda. value
which is higher than 1.
[0026] FIG. 3 is a schematic illustration of a typical response
according to the embodiment in the case of an initial .lamda. value
which is lower than 1.
[0027] FIG. 4 is a schematic flow chart of a method of controlling
a mobile heating device according to an embodiment.
[0028] An embodiment will now be described with reference to FIGS.
1 to 3. FIG. 1 is a schematic illustration of a mobile heating
device according to an embodiment. In the embodiment that will be
described in the following, the mobile heating device 1 is adapted
as a parking heater and/or additional heater for a motor vehicle.
The mobile heating device 1 can for example be realized as a liquid
heater which is adapted to heat a liquid, e.g. cooling liquid in a
cooling liquid circuit of an engine of a vehicle, as a medium to be
heated. As another example, the mobile heating device 1 can also be
realized as an air heater which is adapted to heat air as a medium
to be heated. Mobile heating devices 1 of this kind are well known
in the art and comprise a combustion chamber 2 in which combustion
air and fuel are mixed and combusted in order to release heat which
is used for heating the medium to be heated.
[0029] As schematically shown in FIG. 1, the mobile heating device
1 according to the embodiment is realized as a so-called
evaporation burner comprising an evaporator body 3 for evaporating
liquid fuel which is supplied by a fuel supply 4 (schematically
indicated). The liquid fuel may for example correspond to the
liquid fuel which is also used for the engine of the motor vehicle.
The fuel supply 4 may in particular comprise a fuel pump (not
shown) which is adapted for supplying fuel to the evaporator body 3
with a predetermined fuel supply rate as schematically depicted by
an arrow. According to the embodiment, the fuel supply 4 is adapted
such that fuel can be supplied at different fuel supply rates which
correspond to different heating power levels of the mobile heating
device 1. The mobile heating device 1 further comprises a
combustion air supply 5 (schematically indicated) for supplying
combustion air to the combustion chamber 2 with a combustion air
supply rate, as schematically depicted by arrows in FIG. 1. The
combustion air supply 5 may comprise a fan or blower (not shown)
which can be operated at different speed levels in order to provide
different air supply rates. Both the fuel supply 4 and the
combustion air supply 5 may preferably be adapted to enable
continuously adjusting the respective supply rates. However, it is
also possible to realize the fuel supply 4 and/or the combustion
air supply 5 such that only discrete levels of the respective
supply rate can be set.
[0030] A heat exchanger 6 through which the hot exhaust gases are
guided is arranged downstream of the combustion chamber 2. The hot
exhaust gases from the combustion chamber 2 are guided in contact
with an inner wall of the heat exchanger 6 as schematically
depicted by arrows in FIG. 1. The medium to be heated is guided
between the opposite side of the inner wall and an outer wall of
the heat exchanger 6, as also schematically depicted in FIG. 1. In
the heat exchanger 6, heat is transferred from the hot exhaust
gases via the inner wall to the medium to be heated. After having
passed the heat exchanger 6 the exhaust gases are discharged
through an exhaust gas outlet 7. The mobile heating device 1 is
further provided with at least one temperature sensor which is
adapted and arranged for measuring a temperature which is
characteristic for the combustion process taking place in the
combustion chamber 2. In the embodiment illustrated in FIG. 1, the
temperature sensor is formed by an exhaust gas temperature sensor 8
for measuring the temperature of the exhaust gas discharged from
the combustion chamber 2 of the mobile heating device 1. The
exhaust gas temperature sensor 8 is arranged in thermal contact
with the exhaust gases discharged from the combustion chamber 2.
Although an arrangement of the exhaust gas temperature sensor 8 in
close proximity to the exhaust gas outlet 7 is schematically shown
in FIG. 1, such an exhaust gas temperature sensor 8 may also be
arranged at a different position, e.g. closer to the combustion
chamber 2 with regard to the flow path of the exhaust gases.
Further, although the temperature sensor is realized as an exhaust
gas temperature sensor 8 in the embodiment (which is a preferred
realization), it is also possible to use one or more other
temperature sensors for detecting a temperature which is
characteristic for the combustion process in the combustion chamber
2. Further, it is also possible to analyze the signals of the
exhaust gas temperature sensor 8 and of one or more other
temperature sensors.
[0031] The mobile heating device 1 is provided with a control unit
10 which is adapted for controlling operation of the mobile heating
device 1. To this end, the control unit 10 is connected to the at
least one temperature sensor such that it receives a signal
representing a temperature which is characteristic for the
combustion process in the combustion chamber 2. The control unit 10
is further connected to the fuel supply 4 such that it can control
the fuel supply rate and is connected to the combustion air supply
5 such that it can control the combustion air supply rate.
[0032] In the embodiment, in normal operation the control unit 10
operates the mobile heating device 1 at a certain heating power
level which can e.g. be defined by a certain predetermined fuel
supply rate. For such heating power level, the control unit 10
operates the combustion air supply 5 with a predetermined
combustion air supply rate which is set such that--under normal
conditions--a .lamda. value in a predetermined range is expected
(e.g. 1.2.ltoreq..lamda..ltoreq.1.8) which corresponds to operation
with low emissions.
[0033] A method of controlling the mobile heating device 1
according to the embodiment will now be described with reference to
FIGS. 2 to 4. In FIGS. 2 and 3, the fuel supply rate sr and the
signal T of the temperature sensor (which in the present embodiment
is formed by the exhaust gas temperature sensor 8) are shown in
arbitrary units as a function of time.
[0034] As can be seen in FIG. 2 and FIG. 3, in a first step S1 up
to a time t.sub.1 the fuel supply rate sr (broken line) is
maintained on a constant level sr.sub.0 (initial fuel supply rate)
which corresponds to a predetermined heating power level of the
mobile heating device 1. The combustion air supply rate is not
shown in FIGS. 2 and FIG. 3, but according to the embodiment, the
combustion air supply rate is maintained on a constant level
(initial combustion air supply rate) during the depicted time
period.
[0035] The signal T of the temperature sensor is monitored by the
control unit 10 (step S2). In the present embodiment, the signal T
of the temperature sensor is e.g. sampled every 0.1 seconds or
every second.
[0036] At the time t.sub.1, the fuel supply rate sr is abruptly
reduced to a lower level sr.sub.x, with
sr.sub.x.ltoreq.0.67.times.sr.sub.0, as can be seen in FIGS. 2 and
3. In other words, the fuel supply rate sr is abruptly reduced by
at least one third. At a time t.sub.2 which is several seconds
after the time t.sub.1, e.g. t.sub.2-t.sub.1=10 s, the fuel supply
rate sr is abruptly returned to the initial fuel supply rate
sr.sub.0. Thus, in the embodiment shown, in a third step S3 the
fuel supply rate sr is temporarily changed in the manner of a
square wave signal for a predetermined time period from t.sub.1 to
t.sub.2.
[0037] As can be seen in FIGS. 2 and 3, a clear response to this
temporary change in the fuel supply rate sr occurs in the signal T
of the at least one temperature sensor, in particular if the
temperature sensor is formed by the exhaust gas temperature sensor
8. As can also clearly be seen in FIGS. 2 and 3, the response
clearly differs in the case where the initial .lamda. value was
larger than or equal to 1 (.lamda..gtoreq.1; FIG. 2) as compared to
the case where the initial .lamda. value was lower than 1
(.lamda.<1; FIG. 3). Thus, the response in the signal T of the
temperature sensor can be used to assess if the initial air-fuel
ratio was such that .lamda.>1 or such that .lamda.<1.
According to the embodiment, the control unit 10 is adapted to
analyze the response in the signal T of the temperature sensor and
to assess if the initial .lamda. value was equal to or higher than
1 or if the initial .lamda. value was lower than 1. This is done in
a step S4. Analyzing the response by the control unit can e.g.
encompass analyzing of one or more temperature gradients,
comparison of the signal to pre-set threshold values and the like.
As is evident for a skilled person, many suitable techniques for
analyzing the response in the signal T of the temperature sensor
are available.
[0038] In the embodiment shown, in a step S5, based on the response
in the signal T of the temperature sensor, the control unit 10
operates the mobile heating device 1 with an adapted fuel supply
rate and/or with an adapted combustion air supply rate. For
example, if the analysis results in that the initial .lamda. value
was below 1, the control unit 10 can preferably operate the mobile
heating device 1 with unchanged fuel supply rate but with an
adapted combustion air supply rate which is increased as compared
to the initial combustion air supply rate in order to shift the
.lamda. value to a higher value. Alternatively, if an initial
.lamda. value lower than 1 is found, the control unit could also
judge that a defect in the combustion air supply is present and
shut down operation of the mobile heating device 1. On the other
hand, if an initial .lamda. value which is in the desired range is
found, the control unit 10 operates the mobile heating device 1
with unchanged combustion air supply rate and unchanged fuel supply
rate.
[0039] Now, the reason for the difference in the response in the
signal T of the temperature sensor will be described. First, the
response for the case of an initial .lamda. value equal to or
higher than 1 will be described with reference to FIG. 2.
[0040] If the initial .lamda. value is equal to or higher than 1,
in step S1 the fuel supplied to the combustion chamber 2 is
completely combusted which results in a high temperature in the
combustion chamber and thus a high temperature represented by the
signal T. At the time t.sub.1, the amount of fuel which is supplied
to the combustion chamber 2 is abruptly reduced such that less fuel
for combustion is made available to the combustion process. As a
result, the temperature in the combustion chamber 2 and thus the
signal T of the temperature sensor drops in response to the
temporary change in the fuel supply rate. After the initial fuel
supply rate is restored at time t.sub.2, the temperature in the
combustion chamber 2 and the signal T of the temperature sensor
return to their initial higher values, as can be seen in FIG.
2.
[0041] If however the initial .lamda. value is lower than 1, as
depicted in FIG. 3, in step S1 the fuel supplied to the combustion
chamber 2 is not completely combusted. As a consequence, the
temperature in the combustion chamber 2 and thus the signal T of
the temperature sensor is lower as compared to a case in which
complete combustion would take place. Further, in this case excess
fuel is present in the evaporator body 3 due to the deficiency in
combustion air. At the time t.sub.1, the fuel supply rate is
abruptly reduced such that less additional fuel is supplied to the
combustion chamber 2. Since the amount of combustion air supplied
to the combustion chamber 2 is maintained unchanged, the actual
air-fuel ratio is shifted to a higher .lamda. value. Further, the
excess fuel which is present in the evaporator body 3 is now
evaporated more efficiently. This means that more complete and thus
more efficient combustion takes place which results in an increased
temperature in the combustion chamber 2 as represented by the
signal T of the temperature sensor, as can be seen in FIG. 3.
[0042] Thus, according to the embodiment described above, a method
of controlling a mobile heating device 1 and a mobile heating
device 1 which is correspondingly adapted are disclosed which
enable assessing the air-fuel ratio present in the combustion
chamber 2 in an indirect manner without requiring one or more
additional sensors. In turn, this enables controlling the mobile
heating device 1 such that operation with an increased level of
emissions can be reliably prevented.
Modifications of the Embodiment
[0043] Although an embodiment has been described in which the fuel
supply rate is temporarily reduced by a certain factor while the
combustion air supply rate is maintained constant, it is also
possible to temporarily increase the fuel supply rate while the
combustion air supply rate is maintained constant. This would also
result in a response in the signal T of the at least one
temperature sensor which can be clearly attributed to either one of
an initial .lamda. value of equal to or larger than 1 or an initial
.lamda. value smaller than 1. Preferably, in this case the fuel
supply rate is abruptly increased by a factor of 1.3 or higher
(i.e. by at least thirty percent) in order to achieve a clear
response. Again, a temporary change having the shape of a square
wave signal is particularly preferred.
[0044] Similarly, instead of temporarily changing the fuel supply
rate while maintaining the combustion air supply rate unchanged, it
is also possible to temporarily change the combustion air supply
rate while maintaining the fuel supply rate unchanged. In the case
of an increase in the combustion air supply rate, a similar
response as in the case of a reduction in the fuel supply rate can
be observed. On the other hand, the response to an abrupt decrease
in the combustion air supply rate is somehow similar to the
response to the abrupt increase in the fuel supply rate. In these
cases again, a reduction by a factor of 0.67 or lower or an
increase by a factor of 1.3 or higher is preferred.
[0045] Further, combined schemes for the temporary change in which
both the fuel supply rate and the combustion air supply rate are
temporarily changed are also possible.
[0046] Although an embodiment has been described in which a rather
general distinction is made between, on the one hand, an initial
.lamda. value which is equal to or higher than 1 and, on the other
hand, an initial .lamda. value which is lower than 1, the response
in the signal T of the temperature sensor can be analyzed even more
detailed in order to evaluate the value of the air-fuel ratio more
in detail. Further, it is also possible to analyze the signal of
more than one temperature sensor.
[0047] According to a further development, the step of temporarily
changing at least one of the fuel supply rate and the combustion
air supply rate and analyzing the response can also be repeated
several times. In this case the response can be analyzed by
statistical methods in order to achieve an even more detailed
and/or more reliable result. According to an even further
development, the step of temporarily changing at least one of the
fuel supply rate and the combustion air supply rate is be repeated
several times but with different amounts of change, i.e. with
different intensities. By analyzing the responses to such changes,
the initial .lamda. value can be assessed with high accuracy more
in detail.
[0048] The disclosed method can e.g. be conducted each time the
mobile heating device 1 is started at a predetermined point in time
after start or in regular or irregular intervals during the
life-time of the mobile heating device 1.
[0049] The described method of controlling a mobile heating device
and the correspondingly adapted mobile heating device enable
assessing the quality of combustion in the combustion chamber
without requiring additional sensors. Defects of the combustion air
supply and plugging of the combustion air supply line can reliably
be detected. Further, analysis can be integrated easily in the
control unit and, if desired, more detailed analysis is also
possible.
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